This invention relates to the application of coatings in vacuum and finds particular use in the machine-building industry and other fields of engineering. The method is intended mainly for obtaining the wear-resistive coatings on the parts of machines and tools participating at a reduced temperature (0.degree. to 200.degree. C.).
At present, it is common knowledge that to obtain strong coatings with a satisfactory adhesion to the substrate, it is necessary to maintain the substrate temperature at the level of several hundred degrees. This is especially true for coatings of refractory metal compounds such as titanium and zirconium nitrides. At a temperature of the substrate of less than 300.degree. C., the coatings become fragile and spontaneously separate. That is why, at present, the articles made of structural and carbon steels are not hardened with vacuum-plasma coatings.
A prior art method suggests treating the articles made of conducting materials (U.S.S.R. Inventor's Cert. No. 891217, cl. C23C 15/00). Usually, the surface of any article has a defective layer due to the previous machining. Presence of the defects on the surface reduces the strength of the article as a whole and increases its fragility. The articles are treated with a flux of neutral atoms and ions of the metal having an energy of 0.2 to 2 keV; the amount of ions constitutes 30 to 95% of the total flux. Such a treatment tends to heal the macro and microdefective surfaces, which increases the hardness of the article as a whole.
To perform hardening using the said method, the article is immersed into the vacuum chamber accommodating the electric-arc evaporator, the chamber is depressurized to a high vacuum, a negative voltage of several hundreds of volts is applied to the article, and the evaporator is turned on for three to five minutes. Simultaneous treatment of the article with accelerated ions and neutral atoms of evaporated metal ensures an increase in mobility of the deposited metal atoms, healing of the surface defects and, as a consequence, an increase of the article hardness.
Such ion-plasma treatment of the articles having previously applied wear-resistive coating has not shown a positive effect probably because of the presence of concealed defects in the depth of the coating layer which cannot be reached by the atoms of the healing metal.
The object of the present invention is to increase the wear-resistivity of the coatings.
The object is attained by depositing the coating in the presence of reaction gas while the substrate temperature is maintained in the range of 0.degree. to 200.degree. C., preferably below 100.degree. C. The substrate is subjected to pulses of negative voltage with an amplitude defined generally by the relationship: EQU 0.5U.sub.sl .ltoreq.U.sub.n .ltoreq.4U.sub.sl
with pulse amplitude preferably in the range of 0.5 to 10 kilovolts; with a pulse duration defined generally by the relationship: EQU 10 .delta..sub.o C.sub.o .ltoreq.t.sub.pls .ltoreq.200 .delta..sub.o /C.sub.o
with pulse duration preferably in the range of 0.1 to 25 seconds; and the ratio of pulse period to pulse duration in the range of 2 to 10. In these relationships:
U.sub.n is the substrate voltage; PA0 U.sub.sl is the voltage on the substrate at which the coating deposition rate in the high vacuum without feeding the reaction gas is equal to zero; PA0 t.sub.pls is the duration of pulses in seconds; PA0 C.sub.o is the coating deposition rate in Angstroms per second; PA0 .delta..sub.o is the thickness of monolayer, Angstroms.
Between voltage pulses, a layer of compounds of the evaporated metal and gasified reactant is deposited on the cool substrate; this layer may have many defects. The lower the temperature of the substrate, the more defective this layer may be. When a voltage pulse is then applied to the substrate, bombardment by ions together with the neutral atoms occurring in the process "heals" the defects of the deposited layer, i.e., causes recrystallization, and increases coating hardness. Because of a small duration of the pulse, the substrate has no time to be warmed up. The substrate temperature can thus be maintained within 0.degree. to 200.degree. C. with appropriate cooling.
With increasing substrate temperature, the physical and mechanical properties of the obtained coatings deteriorate; when the substrate temperature exceeds 200.degree. C., the "healing" effect disappears. The layer-by-layer healing of the microdefects ensures a high wear-resistivity of the coating as a whole.
With a voltage pulse duration less than 10 .delta..sub.o /C.sub.o, the process of "healing" the defects, i.e., recrystallization, is incomplete, and the coating remains fragile and insufficiently strong. When the pulse duration exceeds 200 .delta..sub.o /C.sub.o, the temperature of the lower sublayers and the base increases, producing a coating in which the mechanical properties of the coating as a whole are reduced.